LIGHT SOURCES AND DETECTORS 



The 150 or 250 W infra-red spotlight with internal reflector designed for 

 heating hatching chickens can often be usefully employed in the biological 

 laboratory for maintaining the body temperature of anaesthetized animals. 



150 watt 



75 watt 



30° 



25° 



20° 



30' 



25' 



20* 



15° 10° 5° 0° 5° 10° 15° 15° 10° 5° 0° 5° 10° 15° 



Polar diagram candlepower 



Grid filament 

 spotlight 



■Ring filament 

 spotlight 



Floodlight 



Figure 28.9 Polar diagrams of the candlepower distribution 

 of internal reflector type lamps 



Gas-discharge lamps 



With a tungsten lamp running at a high temperature it is difficult to 

 exceed a source luminance of 5,000 cd/cm^. If a higher luminance is required 

 it is necessary to use some form of arc discharge. The following sources 

 are a few of the many available. 



Type M.E. high-pressure mercury vapour lamp — This consists of a quartz 

 bulb with two tungsten electrodes between which can be formed an arc in 

 mercury vapour under high pressure {Figure 28.10). To protect the user 

 against accidental rupture of the lamp it is enclosed either in a metal box 

 with a tough glass window or in a glass envelope. It runs very hot but it 

 should not be cooled by forced air as this reduces its efficiency. If mounted 

 in a box with a concave reflecting mirror the effective luminance of the arc 

 can be nearly doubled, for the arc is transparent to its own radiations and 

 does not obscure the image formed by the mirror as is the case with a solid 

 tungsten filament. 



The arc, when it strikes, acts as a negative resistance so that appropriate 

 supply circuits are required. It will operate on either an a.c. or d.c. supply 

 of 200 V or more, using a choke and resistance to limit the supply current 

 and to provide an inductive starting voltage surge. It takes about 10 minutes 

 for the lamp to reach its maximum light output after starting : if it is then 

 extinguished it wifl not re-strike for another 10 minutes until it has cooled. 



344 



